After losing its ratings lead and falling to last in primetime, the once-dominant CNN stands to lose the last piece of top-shelf value still attached to its business side: premium pricing. Time Warner-owned CNN, while getting beaten handily in the ratings race and having fallen to fourth place in rankings, still commands higher ad rates than rivals -- in some cases double those of Fox News and MSNBC. But perhaps not for long. While advertisers have been willing to shell out more for CNN's venerable brand, broad audience reach and less-opinionated programming, media buyers said the network's ratings slide is...